REGULATION OF GENE EXPRESSION IN THE TRANSITION FROM CELL ELONGATION TO SECONDARY WALL FORMATION IN COTTON FIBER

摘要

During the transition from cell expansion to secondary cell wall thickening, the rate of cellulose biosynthesis in cotton fibers rises nearly 100-fold. Although the gene for the cellulose synthase catalytic subunit, CesA, was first described from cotton fiber, little is known about how CesA expression is regulated. By real-time quantitative PCR (q-PCR) we have identified the group of cotton CesA genes that are expressed during cell elongation and another set of CesA genes that are expressed during secondary wall thickening. The timing of the transition from elongation to cellulose biosynthesis is well-established for fiber cells produced in vitro by cotton ovule cultures. In this study we investigated changes in culture conditions that alter the timing of secondary cell wall CesA expression. Relative gene expression levels were monitored by q-PCR using SYBR Green for detection in an Applied Biosystems 7900HT Sequence Detection System. Gene-specific primers were designed with Primer Express ver. 2.0(Applied Biosystems). Melting curve analyses were conducted to verify primer specificity. Relative transcript levels were determined by a comparative Ct method using either 18S rRNA or cotton a-tubulin 4 as normalizers. Twenty-four hour treatment with exogenous indole acetic acid and/or abscisic acid at a time prior to initiation of secondary cell wall synthesis stimulated the premature expression of CesAl and CesA2, genes responsible for secondary wall synthesis in cotton fiber. Simultaneous treatmentwith auxin and abscisic acid had an additive effect on relative transcript abundance for CesAl and CesA2. Similar phytohormone treatments had little effect on the expression of genes predominantly expressed during cell elongation or constitutively expressed throughout fiber development (i.e. a-tubulin 4, a-tubulin 5, actin, expansin 1, and ubiquitin conjugating enzyme). Furthermore, addition of exogenous gibberellic acid, an essential phytohormone for fiber elongation down-regulated expression of secondary wall CesA genes. Evidence for a similar pattern of phytohormone-mediated gene regulation of a cotton CesA promoter in transgenic Arabidopsis will be discussed with a model that integrates these results.